Efficient, Stable, and Scalable Push-Pull Heptamethines for Electro-Optics

Currently, there is increasing interest in the use ofpush-pull heptamethine chromophores containing the 2-dicyano-methylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) elec-tron acceptor for electro-optic (EO) applications. Compared withthe benchmark push-pull tetraenes with a stronger trifluoromethyl-substituted TCF acceptor (CF3-TCF), the TCF-based push-pullheptamethines showed lower EO activities; however, they havebetter chemical and thermal stability and higher synthetic efficacy.Herein we report the facile synthesis and analysis of structure-property relationship of dipolar heptamethines containing a strongelectron acceptor, bis(4-fluorophenyl)-substituted TCF (FP-TCF).The FP-TCF acceptor was synthesized through two consecutivemultiple steps, one-pot reactions to achieve a good overall yield of50%, which is higher than that of making CF3-TCF. The reduced pi-pi stacking andfine-tuned bond-length alternation boost the nonlinear optical activities of FP-TCF-based heptamethines inplasmon-coupled EO polymer waveguides, leading to larger EO coefficients (up to 205%) over those of the TCF-basedchromophores. More importantly, we demonstrate for thefirst time an ultralarge molecularfirst hyperpolarizability of 8437x10-30esu at 1304 nm forM1a-FP-ON, which exceeds the results of two best-performing but synthetically demanding push-pull tetraenechromophores. The results mark an important milestone in developing highly efficient push-pull polymethines with good stabilityand synthetic scalability for photonic applications.